Side rails B of a conventional frame structure A for a vehicle and particularly for a truck shown in FIGS. 4-7 is formed in such a manner that an inner channel D is engaged with an outer channel C to be welded at flanges E of the outer channel C to the flanges F of the inner channel D at the welded portions G as shown in FIG. 5 to form an integral structure of closed section.
In such an integral structure, the assembling width W of the outer channel C and the inner channel D for forming the side rail B must be maintained constantly. If the width W is not held constantly, the strength of the frame cannot be proved, and the mounting accuracy of other parts to be mounted on the outer and inner channels C, D can be adversely affected, thereby causing the assembling of the vehicle from being disturbed.
In order to maintain the abovementioned assembling width W constant in the conventional frame structure, small holes H, I are respectively opened heretofore at the outer and inner channels C and D as shown in FIG. 6, and a pin J passed from the outer channel C side is inserted to provide means for restricting the interval between the channels C and D. Or, as shown in FIG. 7, a bracket K is welded to the inner channel D, the interval between the channels C and D is restricted by the bracket K, or as disclosed in Japanese Utility Model Laid-open No. 139475/83 official gazette, cutouts are formed at the flanges of the inner channels to form projections having butting surfaces, which are opposed at the edges of the flanges of the outer channel and welded in a structure.
Since the abovementioned means using the pin J has narrow contacting surface of the pin J and the inner channel D, the inner channel D tends to incline so that the accuracy of the assembling width W can not be accurately maintained. Further, it is necessary to provide a number of small holes H, J for passing the pins J, but the many holes cannot be formed without disadvantageously affecting the strength.
In the conventional structure using the bracket K, the weight and the cost increase in the amount of the bracket K, and the step of welding the bracket K to the inner channel D is also required, and yet if the types of the side rail B increase due to the difference in the thickness of the inner channels D, various types of the brackets K must also be prepared for the respective side rails B, and it is also necessary to provide the managing step of preventing the erroneous assembling, which is a drawback.
In addition, in the structure disclosed in Japanese Utility Model Laid-open No. 139475/83, a number of cutouts are formed at the inner channels, resulting in a difficulty in the strength.